Wheel bearing apparatus that freely rotationally supports a wheel of a vehicle includes a wheel hub for mounting a wheel via a rolling bearing for a driving wheel and a driven wheel. Due to structural constraints, the inner ring rotation type is used for the wheel bearing apparatus for a driving wheel and both the inner ring rotation type and the outer ring rotation type are used for the wheel bearing apparatus for a driven wheel. Double row angular contact ball bearings, with low rotational torque characteristics, are popularly adopted in the wheel bearing apparatus in the point of views of having a desirable bearing rigidity. Also, the ball bearings exhibit high durability against desirable bearing rigidity. Also, the ball bearings exhibit high durability against misalignment and improved fuel consumption. In the double row angular contact ball bearing, a plurality of balls are interposed between a secured ring and a rotational ring and contact them while applying a predetermined contact angle onto the balls.
The wheel bearing apparatus are classified broadly into the first, second, third or fourth generation type. In the first generation type, a wheel bearing includes a double row angular contact ball bearing etc. fit between a wheel hub and knuckle forming part of a suspension apparatus. The second generation type includes a body mounting flange or a wheel mounting flange directly formed on the outer circumference of an outer member (outer ring). The third generation type includes one inner raceway surface directly formed on the outer circumference of a wheel hub. The fourth generation type includes the inner raceway surface directly formed on the outer circumferences, respectively, of the wheel hub and the outer joint member of the constant velocity universal joint.
In recent years, there have been strong desires to improve “NVH”, i.e. “Noise”, “Vibration” and “Harshness” to say nothing of improvement of the durability and reduction of manufacturing cost. As shown in FIG. 7, a prior art wheel bearing 50, used in the wheel bearing apparatus, is formed by a double row angular contact ball bearing. It includes an outer member 51 formed on its inner circumference with double row outer raceway surfaces 51a, 51a each having a circular arc cross-section. A pair of inner ring 52, 52 is each formed on its outer circumference with an inner raceway surface 52a having a circular arc cross section opposite to one of the double row outer raceway surfaces 51a, 51a. Double row balls 53, 53 are contained between the outer and inner raceway surfaces. The bearing portion of each row has a contact anglea. A seal 54 is mounted in the annular openings formed between the outer member 51 and the inner ring 52. The seal 54 prevents leakage of lubricating grease sealed within the bearing and the entry of rain water or dust into the bearing from the outside.
In higher shoulder edges 55, 56, in cross-sections of the outer and inner raceway surfaces 51a, 52a, they are formed with auxiliary raceway surfaces 55a, 56a, respectively, smoothly continuous to curved surfaces “a”, “b” of circular arc cross-section. Each of the auxiliary raceway surfaces 55a, 56a has a cross-section formed by a concave line or straight line having a curvature smaller than that of the curved surfaces “a”, “b”. Chamfered portions 55b, 56b, each having a circular arc cross-section, are continuous with the auxiliary raceway surfaces 55a, 56a. 
In such a wheel bearing apparatus, since they are formed with the auxiliary raceway surfaces 55a, 56a, the contact ellipse of the ball 53 would be “pushed out” from each raceway surface 51a, 52a to the auxiliary raceway surface 55a, 56a. However, since the auxiliary raceway surfaces 55a, 56a are continuous with the curved surfaces “a”, “b”, forming the cross-section of the raceway surfaces 51a, 52a and have straight cross-sections, the generation of the edge load (excessive stress) will be prevented even though the contact ellipse would be pushed out.
In addition, each of the auxiliary raceway surfaces 55a, 56a has a straight cross-section. Thus, it is possible to set the inclination of the auxiliary raceway surfaces 55a, 56a larger as compared with an inclination formed by an extension of the circular arc curves “a”, “b” of the raceway surfaces 51a, 52a even though it is set that the inner diameter of the outer member 51 is small or the outer diameter of the inner ring 52 is large. Accordingly, a condition where the auxiliary raceway surfaces 55a, 56a have to be ground using a side surface of a grinding wheel can be avoided. Thus, time for grinding can be reduced.
In addition, the raceway surfaces 51a, 52a, respectively, have the chamfered surfaces 55b, 56b of circular arc cross-section continuous with the edges of the auxiliary raceway surfaces 55a, 56a. The edge load of the contact ellipse can be further reduced (see e.g. Japanese Laid-open Patent Publication No. 52784/2004).
In the described wheel bearing apparatus, the contact ellipse of the ball 53 would override the shoulder portion of the raceway surface if an excessive load is input onto the wheel bearing from a wheel. Indentations generated on the shoulder would cause abnormal noise when a vehicle runs to curve. For solving the problem of indentations caused in the shoulder of raceway surface, it is necessary to increase the shoulder of the raceway surface. However, increasing the height of raceway surface causes problems of increasing the weight of wheel bearing and a reduction of its workability and finally it increases of manufacturing cost. On the other hand, sufficient sealability will not be assured due to a reduction of the cross-section height of the seal 54 by an amount equal to the increase of the shoulder height of the inner ring 52 if it would be increased. In the present specification, the term “shoulder overriding” means a phenomenon where the contact ellipse formed in a contact portion between the ball 53 and the outer raceway surface 51a is pushed out from the corner between the inner diameter of the outer member 51 and the outer raceway surface 51a. This generates the edge load when a large moment load is applied to the wheel bearing.